Graphechon storage tube



Feb. 4, 195s M. D. HARSH 2,822,493

GRAPHECHON STORAGE TUBE mx .QSSN SMU led April 24, 1955 w Qmmw u; ,.....r....d 1.312

- Y INVENTQR. MHUHILED. HHRSH United States ZPatent` GRPHECHON STORAGE TUBE Maurice-D. Harsh, Lancastelgxl'a., assignor, by mesne assignments,ttofthe United States of America as represented-.by the vSecretary of theNavy Application Aprilv 24, 1953,'Se1'ialNo'. 350372" "I-ClaiIIlS.` (Cla 313-89) This invention is` directed. to an. electron discharge deviceand more particularly to a storage tube for the conversion of one type of electrical signal to another type of electrical signal.

The invention relates specifically `to a storage `tube having an insulating target upon which a signal charge pattern is established. The charge pattern is used yto provide output signals from the tube corresponding, to the charge pattern.v The output signals are utilized to provide a visual picture of the pattern. One storageftube' ofv this typeis that which is fully disclosed in U. S. appli# cation Serial Number 29,746 of L. Pensak, filedY May 28 1948. One modification of this 'type of storage tube utilizes a target electrode having. a thin film of insulating material An electron beam is directed against one side of the Ytarget nlm to .provide a charge pattern onthe insulatingfilm. A second electron beam'is scanned1 over the charged surface of the target film to-di'scha-rge the film and to provide output signals from thetube, may be amplified and. fed into a kinescope or viewing tube to provide a visual picture of the charge pattern on the storage tube target.

Such a storage tube consists essentially of a first electron gun, called a writing gun, directed at one side. of the target' electrode and a second electron gun, known` as the reading gun, directed at the opposite side'of the target. Both of the guns are substantially on the same axisand the target electrode is positioned intermediate the guns an'd substantially normal thereto. The target is formed` with a supporting mesh screen coated lon one side Vwith an electronpervious aluminum film. A thin filmV of insulating material is supported' onthe exposed surface of the aluminum film. v

The reading gun faces the insulating' film surface and provides-a relatively low velocity beam which is scanned over the insulating target film. Secondary electrons are knocked out of the insulating lm surface whenthe reading beam impingcs on the surface and are collected on a conductive coating on the wall of the tube,thus causing the target surface to become charged to an equilibrium potential which is positive with-.respect to the aluminum film. There is an output from the `tube even though no signals are being written. onto the target by .the writing. gun. This signal is referredv to as the background signal and is due to the-secondary'electrons driven off. Ithe target surface by ythe'electron..beam of thefreading. gun. However,y only asnmany secondaries can lleave the surface Aasfare replaced-by the impingiug electron beam of .thereading gun. Y

The Writing gunI provides ahigh velocity beam which, when modulated, for instance, penetrates through the aluminum and insulating films and discharges the surface ofthe insulating film to the potential of the -aluminum tlm at all points penetrated. Thus, when the writing Agun-beam is modulated by asignal applied to the gun, there is established onthe target surface a charge pattern corresponding `to "the incoming signals Vapplied i to 'the writing gun'. Thereadinggun then, upon 'scanning the charged target surface, charges the surface from point to pointl back .to the equilibrium potential and by an amount-'depending on the potential' at each point'set by the writing gun. This charging of the target surface provides successive signal pulses in 'the target circuit,y

which can be amplified andfed to the kinescope or view? ing tube" so that the charge pattern on the storage tube is visually represented. The collection of secondary elec: trons from portions of the'target which are notdischarged by the writing beam, provide the background signal or black leveli of the picture on the kinescope. This co1- l'ectiomof secondaries from these areas should be uniform to providea` uniform blacklevel of the kinescope picture. l

However; tliesignal output'v from such a storage tube has al 'non-uniform black level or background which causes`I a shading or decrease in contrast of the picture. The background shading asignal is` higher in the center of thcpicture Ythan at' the edge. This higher background signal? in thecentralzpart of thel target results from the fact that more secondary electronsv are collected from portions-oftl're'targetnear the center which are not discharged bythe'fwritingbeam than are collected from target portionsn'ear' the'edgewhich are not.l discharged by' the writingbeam. Themetalli'c ring for supporting the target'is"normally=operated at 'about 40 volts negative with'respect to thel 'collector electrode, and it has beenedeterminedi that this neagtive potential, sets up a retardingfield'betwee'rr thetarget and the conductive coatingeory collector near the outer 'edgesI of the target so that secondari'es generatednear tlreedge of the-target tend to be directed towards the central part ofthe target and; collected by' the positive areas-"there, This rain of secondaries chargesl theV positive VareasV negatively below equilibrium, and therefore produces a non-uniformity in the background signal". vFringe 'fields from the deflecting yokes are present'also; When the deflecting fiel'dfis` such that the beam is neazt` the edgev ofthe target, this fieldy is in the direction to Acause secondaries leaving thetargetto turn towards the'center'ofthe'target. Thus, in two ways, secondary electrons are forced towards the iniddleof the`- target.

The maximum current from a Written signal 'is fixed by theI potential ofthe targetan'd by the beam current in the writing gun,A and is independent ofV shading. The usable output signal of `the tube isequal to the difference between thesignal'resultingfrom the penetrationofthe writing beam through the target surface 4andthebackground signal.

A reduction ofthebackground or shading signaL'therefore, results in a more usable output signal having improvedl contrast and relativelyA higher amplitude.

A principal object of this invention is 'to 'provide an improved storage tube.

Another object of 'this invention vis tofprovide a storage tube in which the background signal level vis substantially uniform irrespective of the part. of the target where said output signal is derived.

Still another object of this invention is to'provide a storage tube in which the ratio between the modulation signal amplitude and the amplitude storage signal are more-'nearly constant'over theentire `surface ofthe target on which information is stored.

The above and related' objects are `aclievedin accordance'with the present invention by the addition 'of a rshield ring vto the target structure. Theshield ring, insulated from the target and extending along the longitudinal axis of the'tube in the direction of the conductive wall coating or collector on the reading end of the tube ,is operated at a positive potential to overcome rthe previously men` tioned repelling fields which tend to deflect secondary electronstowardsthe "centerof the insulating film. Thus,

the rate of collection of electrons on the conductive wall coating or collector electrode is substantially uniform regardless of which area of the target from whence the electrons were emitted. Y

The invention will best be understood by reference t the following description taken in connection with the ac companying drawing, in which:

Figure l is a cross sectional View of a storage tube utilizing this invention;

Figure 2 is an end view of the target assembly of the tube shown in Figure 1, and

Figure 3 is an enlarged partial sectional view takenl along line 3-3 of Figure 2.

Figure 1 shows an electron discharge device consisting of a storage tube having a tubular envelope portion 10. At one end of the envelope there is mounted an electron gun 12 to be described as the writing gun of the tube. At the opposite end of the envelope there is mounted an electron gun 14 to be known as the reading gun of the tube. Intermediate the electron guns 12 and 14 there is positioned a target electrode 16positioned transversely to the axes of the electron guns l2 and 14.

The storage tube 10 is a type which has been fully described in the copending application of Pensak cited above. The writing gun 12 consists principally of a cathode electrode 18 mounted within a cup like control grid electrode 20 and coaxially aligned with a first accelerating electrode 22 and the second accelerating electrode 24. Electrodes 20, 22 and 24 are mounted successively along a common axis by any well-known means such as glass support rods 26, only one of which is shown for simplicity of illustration, sealed to metal studs projecting from each electrode, as shown. Cathode 18 consists primarily of a closed cylinder with the closed end facing target 16 and being coated with an electron emitting material. The electrodes 20, 22 and 24 have apertures therethrough on the common axis of the gun to provide a passage for electrons emitted from the cathode coating. During tube operation, the cathode emitting surface is heated to provide a copious emission of electrons which are drawn by the positive field of the accelerating electrode 22 away from the cathode surface. These electrons pass through the apertures of electrodes 20, 22 and 24, while the fields between the electrodesform the electrons into a beam, which is brought to a minimum cross sectional area, or spot, on the surface of target 16. The final focusing of the electron beam of gun 12 on target 16 is provided by the field established between the tubular electrode 24 and a conductive wall coating 30 applied to the inner surface of the tube envelope between gun 12 and the target 16. The parts of the electron gun 12 are conventional and the electron optics of such a gm and its operation are well-known and are not further described as they do not constitute a part of this invention.

As shown in Figure 1, the several electrodes of the gun 12 are connected to a voltage divider 2S for providing operating voltages to the respective electrodes of the gun.

A set of voltages is indicated in the drawing. These voltages represent values which have been used in the suc cessful operation of a tube of the type described, but are not meant to be limiting.

Electron gun 14, the reading gun, includes a cathode electrode 32 and a control grid electrode 34 similarly respectively to electrodes 18 and 20 of the writing gun 12t Mounted successively along the common axis betweenl the control grid 34 and the target electrode 16 are an accelerating electrode 36, a focusing electrode 38 and a second accelerating electrode 40. Accelerating electrodes 36 and 40 are connected directly to each other through a common supporting metal cup or gun holder 42. The cathode-control grid assembly of gun 14 is rigidly mounted to the accelerating electrode 36 by means of glass support rods 44, only one of which is shown for simplicity of ilf lustration. A conductive coating/46 extends over'the inner surface of the envelopell) 'from thefaccelerating' 4 electrode 40 to a point adjacent to the screen target electrode 16.

As indicated in Figure 1, the several parts of gun 14 are also connected to external sources of potential for providing operating potentials to the gun electrodes. During tube operation, the gun 14 forms an electron beam with the electron emission from the cathode 32, which is directed onto the target 16 and focused to a fine spot at the point it strikes the target.

Ihe beams emanating from guns 12 and 14 are respectively scanncd over opposite surfaces of target 16 by deecting yokes 48 and 50. These deflection yokes are of conventional design, in that they consist of two pairs of deecting coils mounted in the yoke with the coils of each pair on opposite sides of thetube envelope 10. The coils of each pair are connected in series to a source, not shown, of saw toothed currents for providing a magnetic field in the path of the respective electron beam. The fields of each yoke 48 or 50 provide line and frame scansion ofthe respective electron beams over the surface of target 16. Such deflection coils and their mode of operation is well-known in the prior art and since they do not constitute a part of this invention, they are not described in further detail.

The envelope 10 of the tube is formed from several tubular sections. The portion, for example, enclosing the gun 12 is formed from a pair of glass tubular members 52 and 54 which are connected together through a metal alignment cylinder S8 sealed thereto. In a similar manner the portion of envelope 10 enclosing gun 14 is formed from a pair of tubular glass members 60 and 62 which are joined together through an intermediate alignment cylinder 64.

The tube may be enclosed in a housing of the type disclosed and claimed in co-pending application Serial No. 312,004, filed September 29, 1952, by Fadner, Dyall and Harsh.

Target electrode 16 is mounted on a flat ring 66, to which the tubular envelope portions 54, and 60 housing the respective guns 12 and 14 are both sealed to close the envelope. Target 16 consists primarily of a thin electron pervious metallic film, such as an aluminum film 68, coated on its surface facing gun 14 with a thin film 70 of insulating material such as magnesium fluoride, for eX- ample.

, The operation of the tube is substantially that in which input signals are applied to the control grid 20 of gun 12 to modulate the electron beam of the gun while it is scanned over the surface of target 16. The aluminum target lm 68 is maintained in the order of a -40 volts negative with respect to ground. The electron beam of gun 14 is unmodulated and is scanned over the surface of the magnesium fluoride film 70 in a substantially rectangular raster formed by a normal frame and line television scansion. The beam of electron gun 14 will strike the dielectric target lm 70 at energies in the order of 1,000 volts. These energies are between the first and second crossover potentials for the magnesium fluoride screen so that at every point on target film 70 where the beam of gun 14 strikes, the ratio of the secondary electron emission current from the film to the incident electron current is greater than unity. The secondary electrons are drawn away and collected by the positive collecting coating 46. 4 The loss of secondary electrons from each point of film 70A under the scanning beam of gun 14 raises the -potential -of that point positively to a level close to the potential of coating 46 which is held at ground potential. In this manner then the target surface scanned by the beam of gun 14 is raised to an equilibrium potential which is that potential level at which the secondary electrons leaving the target surface are equal in number to the primary electrons of the beam of gun 14 striking the surface. The modulated electron beam of "gun 12 strikes the target with `energies in the order Vof 9,000 volts, which is sufficient to-cause the electrons of the :beam ofgun 12 to penetrate through the target films 68 and70 as described in the above cited copending application of L. Pensak. The magnesium uoride film 70 is a material which is nnormally insulating, but which becomes conducting when struck by high velocity Yelectrons at those points where the electrons penetrate substantially through the magnesium fiuoride film. Thus, during tube operation, the modulated high velocity beam of gun 12 provides conductivity between the aluminum film 68 which is at a potential of, for example, -40 volts and the exposed 4scanned surface of the magnesium uoride film 70 at equilibrium potential, i. e. close to that of ground potential of the collecting coating 46. The conductivity 'induced at any one point through the magnesium fluoride film 70 is proportional to the strength of the beam of gun 12 inducing the conductivity. Thus, the beam of gun 12 will discharge the positive surface of film 70 toward the negative potential of the aluminum film 68 and in proportion to the strength of the beam inducing the conductivity at each point. The scanning of the high velocity modulated beam of gun 12 over the surface of target 16, in this manner, establishes on the exposed surface of film 70 a negative charge distribution or pattern corresponding to the sequential input signals applied to the control grid 20 of gun 12.

The beam of gun 14, on scanning over the charged surface of the film 70, will instantaneously charge the areas that have been discharged by the beam of gun 12 back to the equilibrium potential. VWhen the beam of gun 14 strikes a negative area of film 70 the secondary emission from this point is instantaneously greater than from a positive area. This instantaneous charging of any one point of Elm 70 results in a corresponding pulse in the circuit 56 of aluminum film or plate 68 causing a change in the voltage across the high resistance 72 of the plate circuit. This voltage is detected and amplified in wellknown manner to provide an output signal from the tube, which is fed to a monitor kinescope or other utilization means. Thus, as the beam of gun 14 scans the charged pattern established by the beam of gun 12 on film 7 0, there is provided a succession of signal pulses which vary in amplitude as determined by the potential of the areas charged.

Bombardment of the target 16 by the writing beam produces a flow of current in the target circuit 56 which must be separated from the target current produced by the reading beam. The separation of the two components of :target current may be accomplished by a time-sharing method, or, as illustrated in Fig. 1, by the use of radiofrequency modulation of the reading component in conjunction with a tuned output amplifier.

If the beam of gun 14 is scanned over the charged surface of target film 70 in a normal television scan, the output is a television signal which will produce on the monitor kinescope a scanning visual representation of the charge pattern written on film 70 of the storage tube. By controlling the beam current of gun 14, it is possible to control the storage time of the charge pattern on film 70. Thus, if the beam current of gun 14 is sufficiently small, the charge pattern on target film 70 may remain for a determinable length of time before it is entirely erased. In this manner then, the scanning rate of the beam of gun 12 may be different from that of the beam of gun 14. Furthermore, the pattern or scanning raster established on film 70 may be written down differently than the reading raster of gun 14. For example, the writing gun 12 may be operated to provide a radar raster of the charge pattern which is laid down on the target 70 in several seconds, while simultaneously the pattern may be continually read off by the reading gun 14 with a television scan lframe time of j/30 of a second.

The storage tube and its operation, as thus described, are in accordance with prior art practice.

However, the metallic target ring is normally operated at, for example, 40 volts negative potential with respect uniformity of the background signal.

4to-tlie*conductiverfcoating or collector electrode -whi'cliis .emission from near the edges of the insulating film-which would normally be attractedto the collector`electrode-46 vis repelled by the negative field-of the target ring and falls back onY the positive center portions of .the insulating film rather than be -collected by the collector electrode 46. This non-uniform collection of electrons, as mentioned before, causes a shading effect in the picture due to non- Also, amplitude of signals due to modulation of the writing beam and signal storage are reduced near the edge of the insulating film asa result of this-nonuniform electron redistribution.

Fringe fields from the detlecting yokes also tend to cause secondary electrons .leaving the edge .of the `target to turn towards the center of the target.

In accordance with the present invention, and as shown more clearly in Figures .2 and 3, the target 16 is provided with a Yring-like shield 74 extending axially towards the reading gun 14.

'The target 16, as previously mentioned, comprises .a thin electron pervious metallic (aluminum, for example) film 68 on a fine mesh carrier 88. The mesh 88 is secured to the mesh target supporting member or ring 76 by the target retaining mesh welding ring member 78. The film of insulating material 70 is deposited on thesurface of metallic 'film 68 which faces the readingrgun 14.

The ring-like `shield 74, which faces the reading `gun '14, is secured to a shield support ring or member .80

'the target 16. This method of-mounting the targetassembly in the tube offers several advantages over Apriorart target assembly mounting arrangements. First, the .entire target assembly may be completed .outside the .tube where the alignment of parts may be accurately controlled. Formerly, target assemblies for tubes of the type described have been secured to the tube envelope by glass ybeadiseals, and the heat required to effect these seals is so intense that damage to the thin target would almost certainly occur. Therefore, in the past, only a target holder member would be sealed to the envelope Wall, and the remainder of the assembly was completed inside the tube envelope. Naturally under such circumstances it is very difiicult to accurately mount the target perpendicular to the longitudinal axis of the tube.

In target assemblies in accordance with the present invention, sealing the target in a plane perpendicular to the longitudinal axis of the tube is a simple matter, as the three tabs of the target assembly rest on, and are welded to, the fiat ring 66 which has previously been accurately positioned with reference to the tube axes.

Electrical contact to the shield 74 is made by a spring wiper contact 84 which brushes against a conductive coating on the inner surface of the envelope wall. A pin (not shown) through the envelope wall may provide a means by which a potential may be applied to the shield 74.

Since the negative potential of the target ring 76 and fringe fields from the deecting coils repel some of the secondary electrons emitted from the coating of insulating material 70, especially secondaries emitted from near the edges of the target 16, and causes these electrons to fall back on the central portion of the target, a positive potential is applied to the shield 74 from a circuit 86. The positive potential on the shield 74 is adjusted to overcome the effect of the deflecting fields due to both the negative ring 76 and the deecting field of yoke 50. The

result is that the background is substantially uniform over the entire target and shading is eliminated.

In a typical storage tube of the type under consideration herein, it was found that a positive potential of about 40 volts on the shield 74 produced excellent results when radio frequency modulation of the reading beam was used. A positive potential of about volts on the shield 74 proved suiiicient to provide uniform output when the reading beam was un-modulated.

Since the addition of the shield ring 74 adds to the capacitance of the target assembly, it is desirable if radio frequency signal separation is used to isolate the shield ring 74 from associated wiring capacitance. A resistor 92 of about one half megohm value has been found to be satisfactory for use as the isolating impedance.

Incorporation of the present invention in storage tubes of the type described has resulted in a shading-free target and in a 2-to-l improvement in the signal-to-noise ratio.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A target assembly for a storage tube comprising a target structure and a shield structure, said target structure including an annular target supporting member, a sheet-like target, and an annular retaining member, said target being stretched across said target supporting member, a peripheral part of said target being secured between said target supporting member and said retaining member, said shield structure comprising a tubular shield and an annular shield supporting member, said shield being secured to but insulated from said shield supporting member, said retaining member of said target structure being secured to said shield supporting member of said shield structure.

2. A target assembly for a storage tube comprising a target structure and a shield structure, said target structure comprising an annular target supporting member, a target including a mesh carrier, and an annular retaining member, said mesh carrier being carried by said target supporting member, a peripheral part of said mesh carrier being secured between said target supporting member and said target retaining member, said shield structure comprising an annular shield and an annular shield supporting member, said shield being secured to said shield supporting member by an insulating fusion type seal, said retaining member of said target structure being secured to said shield supporting member of said shield structure, and means for biasing said shield with respect to said target.

3. An assembly for a storage tube comprising a conductive mesh, 'a thin metallic film supported on said mesh, an insulating coating supported on said metallic film, an annular target supporting member, an annular retaining member, said mesh being stretched across said target supporting member, a peripheral part of said mesh being secured between said target supporting member and said retaining member, an annular shield supporting member secured to said retaining member, and a tubular anged shield secured to but nuslated from said shield supporting member with said shield on the same side of said assembly as said insulating coating.

4. An electron discharge device for the storage of electrical signals comprising an elongated envelope, an electron beam-producing means in one end of said envelope, a disk-like target assembly within said envelope and in the path of the electron beam from said beam-producing means, means for scanning said target assembly with said electron beam, a target supporting member, a mesh carrier extending across said target supporting member, a target retaining member, a peripheral part of said mesh `carrier being secured between said target supporting member and said target retaining member, a shield structure for providing a substantially uniform background signal level from all parts of said target when said target is struck by said beam, said shield structure comprising a shield and a shield supporting member, said shield being secured to said shield supporting member, said retaining member of said target structure being secured to said shield supporting member of said shield structure, and means for biasing said shield with respect to said target.

YReferences Cited in the iile of this patent UNITED STATES PATENTS 2,493,539 Law Jan. 3, 1950 2,495,042 VWilder et al. Jan. 17, 1950 2,567,359 Anderson Sept. ll, 1951 2,582,843 Moore Jan. 15, 1952 2,590,764 Forgue Mar. 25, 1952 2,598,919 Jensen June 3, 1952 2,611,100 Faulkner et al Sept. 16, 1952 2,644,855 Bradley Iuly 7, 1953 2,646,521 Rajchman July 21, 1953 2,706,264 Anderson Apr. 12, 1955 

